96 SOME PRINCIPLES AND METHODS OF ROCK ANALYSIS, [bull. 176. 
in oxidizing to V 2 5 requires as much oxygen as four molecules of 
FeO (288) when oxidized to Fe 2 3 . The proportion, 150.8 : 288 : : V 2 3 
present : a?, therefore gives the figure to be deducted from the uncor- 
rected value for FeO. That this correction is very needful with many 
of the basic rocks becomes at once evident from the following perhaps 
extreme example: 
Found 2.50 per cent apparent FeO in a rock containing .13 per cent V. 2 O s . 
Deduct .25 per cent FeO equivalent in its action on KMn0 4 to .13 V 2 8 . 
Leaving 2.25 per cent FeO corrected. 
Found 5.00 per cent apparent total iron as Fe 2 3 in the same rock. 
Deduct .14 per cent Fe 2 3 corresponding to .13 per cent V. 2 3 . 
Leaving 4.86 per cent corrected total iron as Fe 2 3 . 
Deduct 2.50 per cent Fe 2 3 equivalent to 2.25 per cent FeO. 
Leaving 2.36 per cent Fe 2 3 in the rock. 
Failure to correct for the vanadium in both cases would have made 
the figures for FeO and Fe 2 3 , respectively, 2.50 and 2.22 instead of 
2.25 and 2.36 as shown above. 
Carbonaceous matter. — As said before (p. 94), matter of organic 
origin other than graphitic carbon renders the results of the ferrous 
iron determination altogether unreliable. 
UNCERTAINTIES OF THE FERROUS IRON DETERMINATION. 
From the foregoing it is apparent that, despite the utmost care in 
practical manipulation, the exact estimation of ferrous iron in rocks 
is one fraught with extraordinary difficulties and uncertainties. Only 
in absence of decomposable sulphides, and when the amount and con- 
dition of vanadium are known, can the result be regarded as above 
suspicion. 
XX. ALKALIES. 
THE LAWRENCE SMITH METHOD. 
The various methods for getting at the alkalies in insoluble silicates 
differ more in the mode of attack of the mineral powder and in the 
immediately subsequent treatment than in the final stages. With very 
few exceptions, since the early days of the Survey's existence, all alkali 
determinations have been made by the method of J. Lawrence Smith, 1 
which is far more convenient than and fully as accurate as those in 
which decomposition is effected by hydrofluoric and sulphuric acids, 
or by bismuth, lead, or boric oxides. One of its chief advantages is the 
entire elimination of magnesia at the start. 
Decomposition of the powder is effected by heating it with its own 
'Am. Jour. Sci.,2d series, Vol. L, p. 209,1871; Am. Chemist, Vol. I, 1871; Annalen Chem. und Pharm., 
Vol.CLIX,p.82,1871. 
